their length.
Without this, an error frame mbuf would:
* have its size adjusted;
* thrown at the radiotap code;
* then since it's never consumed, the rxbuf/mbuf is then re-added to the
RX descriptor list with the small size;
* .. and the hardware ends up (sometimes) only DMA'ing part of a frame into
the small buffer, chaining RX frames together (setting the more flag).
I discovered this particular issue when doing some promiscuous radiotap
testing; I found that I'd occasionally get rs_more set in RX descriptors
w/ the first frame length being very small (sub-100 bytes.) The driver
handles 2-descriptor RX frames (but not more), so this still worked; it
was just odd.
This is suboptimal and may benefit from being replaced with caching
the m_pkthdr_len and m_len fields, then restoring them after completion.
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